The keyboard in the unpatched firmware has a keypad of extended Latin letters that is accessed by tapping the
ÉÀÇ key:
It is possible to change the extended Latin letters to other letters with a custom keyboard patch. There are patches for Cyrillic, Arabic, Hebrew, and Turkish, or you can make up your own keypad just by replacing the letters in one of these patches:
Cyrillic (lower and upper case) and Arabic:
Note that the glyphs for Arabic and Hebrew are not available when the device first boots, so the keyboard will just show blank squares until the first time the Aa menu inside a book is opened.
The screenshots above are from my Glo (fw 3.16.0), the keyboards on other devices are a little different.